Mesomorphic or liquid crystal compounds are of increasing interest in a variety of electro-optic display devices. Nematic liquid crystals are of particular interest for electrically controllable, flat panel displays such as watch faces, digital clocks, calculator displays, numeric displays for instruments and the like. Typically, a liquid crystal cell comprises a thin layer of a liquid crystal composition sandwiched between two closely spaced parallel conductive plates, at least one of which is transparent. When the conductive plates are connected to a source of current, an electric field is generated in the liquid crystal composition.
Nematic liquid crystal cells can operate in a dynamic scattering mode, as is described in U.S. Pat. No. 3,499,112 to Heilmeier and Zanoni, or in a field effect mode. Field effect devices contain nematic compounds or mixtures of compounds having positive dielectric anisotropy, that is, the magnitude of the dielectric constant in a direction parallel to the long axis of the molecular chain is greater than the magnitude of the dielectric constant in a direction perpendicular to the long axis of the molecular chain, between conductive plates that have been treated so that the liquid crystal molecules align themselves in a particular direction, usuallly parallel, to the plane of the plates. When an electric field is applied, the positive dielectric anisotropy of the molecules causes the molecules to realign themselves in a direction parallel to the applied field and perpendicular to the plates. The change in alignment is made visible using a polarizer and an analyzer on either side of the cell. Field effect liquid crystal cells have the advantages of lower threshold voltages and wider viewing angle than dynamic scattering cells and have excellent contrast and long lifetimes.
Each mesomorphic compound has a particular temperature range in which it is an ordered liquid, ranging from the solid to nematic liquid crystal melting point up to the temperature at which it forms an isotropic liquid. This is the temperature range useful in electro-optic cells. Although, as is known, wide variations in use temperature can be effected by employing mixtures of liquid crystal compounds that are compatible with each other, no single liquid crystal compound or mixture of compounds can satisfy all use temperature ranges desired. Thus, new liquid crystal compounds which have different use temperature ranges are being sought to satisfy various temperature requirements for which the liquid crystal cells will be employed.